1. Field of the Invention
The present invention relates to a coating deposition method based upon colloidal processing technology.
2. Description of Related Art
A coating layer on a substrate, such as a ceramic film (i.e., coating) deposited on a metal or oxide substrate, can be obtained by several methods. Generally such films can be deposited using methods either requiring or not requiring vacuum technology.
Contemporary vacuum deposition techniques can be grouped into two categories: physical vapor deposition (such as sputtering, laser ablation, etc.) and chemical vapor deposition. Both technologies require expensive vacuum pumping equipment. Because of the relatively high cost of capital equipment, such methods are usually not economically viable for high volume applications.
Physical vacuum deposition methods are also limited because the are “line-of-sight.” That is, deposition only occurs on the surface of the substrate which can be “seen” by the source. Substrates having a more complex geometry than planar typically are poorly coated, if at all, in a vacuum deposition system. Complex geometrical substrates may be rotated and turned in a vacuum system to achieve more complete surface coverage, although this adds considerable complexity to the system. Chemical vapor deposition is more conformal; however, it often uses toxic and/or expensive chemical reactants. Both physical and chemical deposition techniques generally have low deposition rates for oxide films, typically less than 1 micron per hour.
Contemporary non-vacuum methods of applying coatings to substrates include plasma spraying, tape casting; tape calendering; screen printing; sol-gel coating; colloidal spin or dip coating; electrophoretic deposition; slurry painting; and spray pyrolysis coating. Tape casting and tape calendering are generally limited to planar substrates only. Plasma spraying, slurry painting, and screen printing techniques usually yield coatings with almost certain porosity and are thus more appropriate for applications where a fully dense film is not required. Spray pyrolysis, in which a solution of metal salts or organometallics is sprayed on a heated substrate also generally yields porous films.
Colloidal techniques (spin coating, dip coating, and electrophoretic deposition) are among the most cost-effective techniques known for deposition of dense thin films. These techniques involve the preparation of a colloidal solution of the ceramic powder of the material to be coated.
In the spin coating method, a few drops of the colloidal solution is placed on the surface of the substrate, which is subsequently spun at high speed thereby removing the solvent and leaving a thin layer of the powder on the surface of the substrate. This technique is limited to deposition onto planar substrates having low surface areas.
In electrophoretic deposition, a high voltage is applied between the substrate and a counter electrode, both of which are immersed in the colloidal suspension. The powder particles, which are generally slightly charged on the surface, move under the electrostatic potential toward the substrate where they discharge and deposit. This technique is limited to conductive substrates only.
In the dip coating process, the substrate is dipped into the colloidal solution followed by withdrawal and drying. During the air-drying step, the solvent evaporates, leaving the powder in the form of a thin film on the substrate.
It has been extremely difficult, if not impossible, to deposit coatings with thicknesses larger than a few microns, using conventional dip coating methods. The films obtained are generally limited in thickness, typically a few microns, but less than ten microns. Attempts to deposit thicker coatings have not generally been successful because of film cracking, particularly during the drying process. The drying step in a conventional colloidal dip coating process is done after withdrawing the substrate from the solution. During the drying step the solvent evaporates which induces film shrinkage due to a large volume change which in turn leads to cracking. In order to deposit coatings thicker than 10 microns, the coating process must be repeated, which is both time consuming and costly.
In addition, all the colloidal processing techniques require subsequent sintering at high temperature in order to densify the film. The process of thermal cycling of the substrate from room temperature to the sintering temperature, can cause cracking between the successive layers because of differential rates of thermal expansion.
Accordingly, a need exists for coatings on substrates that can be relatively dense, are essentially crack-free, yet sufficiently thick (i.e., greater than 10 μm), and preparable in a single dispersion step.